Gas sensor

ABSTRACT

A gas sensor capable of reversibly and continuously measuring the concentration of a catalyst poison gas such as CO without specially needing recovering means such as a heater, and measuring the catalyst poison gas concentration without being affected by H 2 O concentration. The electrical circuit ( 15 ) of the gas sensor has an AC power supply ( 19 ) for applying an AC voltage between both electrodes ( 3 ), ( 5 ), an AC voltmeter ( 21 ) for measuring an AC voltage (AC effective voltage V) between the both electrodes ( 3 ), ( 5 ), and an AC ammeter ( 23 ) for measuring a current (AC effective current I) running between the both electrodes ( 3 ), ( 5 ). An impedance is determined from the AC effective voltage V and the AC effective current I generated when the AC voltage is applied to the both electrodes ( 3 ), ( 5 ). Since this impedance corresponds to the catalyst poison gas concentration, the catalyst poison gas concentration can be determined from the impedance by using a map showing the relation between the impedance and the catalyst poison gas concentration.

TECHNICAL FIELD

The present invention relates to a gas sensor suitable for measurement,in a fuel cell, of concentration of a catalyst poison gas, such as CO orsulfur-containing substance, contained in fuel gas, particularly,concentration of CO.

BACKGROUND ART

With global-scale environment deterioration being perceived as aproblem, in recent years, there have been actively performed studies onfuel cells, which are highly efficient, clean power sources. Among them,a polymer electrolyte fuel cell (PEFC) is a promising fuel cell, becauseit has advantages of low operation temperature and high output density.

A reformed gas of gasoline or natural gas shows promise as a fuel gas tobe used in a PEFC. However, since CO is generated in the course ofreformation reaction in accordance with conditions such as temperatureand pressure, CO is present in a reformed gas. Further,sulfur-containing substances contained in the crude material may remainin a reformed gas.

Catalyst poisons such as CO and sulfur-containing substances poison Ptor the like, which is a fuel electrode catalyst of a fuel cell.Therefore, demand exists for a gas sensor capable of directly detectingthe concentrations of CO and sulfur-containing substances contained in areformed gas. In particular, the necessity of a CO sensor is high, andsuch a CO sensor is required to be capable of performing measurement ina hydrogen-rich atmosphere.

In view of the above, conventionally, there has been proposed a carbonmonoxide sensor whose detection portion is disposed in a gas to bemeasured (hereinafter referred to as “analyte gas”) and which obtains COconcentration from the gradient of a change in current which flows uponapplication of a predetermined voltage between two electrodes (seePatent Document 1).

Further, there has also been proposed a CO gas sensor which obtains COconcentration from a CO-concentration-attributable change in responsecurrent at the time the applied voltage is changed by a pulse method(see Patent Document 2).

[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No.2001-099809 (page 2, FIG. 1)

[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No.2001-041926 (page 3, FIG. 2)

However, in the technique of Patent Document 1, since CO concentrationis obtained from the gradient of a change in current which flows betweentwo electrodes, a change in current attributable to CO; i.e., a changein the electrode catalyst attributable to CO poisoning, is irreversible.As a measure against this problem, the carbon monoxide sensor hasrecovery means which uses a heater. However, the sensor has a problem ofhaving a complicated structure.

Moreover, in the carbon monoxide sensor, since the current flowingbetween the two electrodes changes depending on the resistance betweenthe electrodes, the gradient of a change in current, which is the sensoroutput, changes with H₂O concentration. Therefore, when the H₂Oconcentration within a measurement atmosphere changes because of, forexample, a change in operating conditions, the sensor output isinfluenced by the H₂O concentration, so that the sensor encountersdifficulty in accurate measurement of CO concentration.

Meanwhile, in the technique of Patent Document 2, CO concentration ismeasured through repeated and alternating application of a CO adsorptionpotential and a CO oxidization potential. However, since COconcentration cannot be measured during periods in which the COoxidization potential is applied to the sensor, the sensor has a problemin that the sensor cannot perform continuous measurement of COconcentration.

Moreover, as in the case of the technique of Patent Document 1,according to this technique, the current flowing between the twoelectrodes changes depending on the resistance between the electrodes;

therefore, the sensor has characteristics such that when the H₂Oconcentration of an analyte gas changes, the gradient of a change incurrent, which is the sensor output, also changes. Therefore, when theH₂O concentration of the analyte gas changes because of, for example, achange in operating conditions, the sensor output is influenced by theH₂O concentration, so that the sensor encounters difficulty in accuratemeasurement of CO concentration.

Furthermore, according this technique, a CO-concentration-attributablechange in hydrogen oxidation reaction at catalyst of an anode electrodeis measured from a change in DC current flowing through solidelectrolyte film, and the CO concentration is obtained on the basis ofresults of this measurement. Since H₂O concentration in the vicinity ofthe catalyst of the anode electrode decreases as a result of the DCcurrent flowing through the solid electrolyte film, desorption of CObecomes less likely to occur, whereby responsiveness is lowered.

An object of the present invention is to provide a gas sensor whichenables reversible, continuous measurement of concentration of acatalyst poison gas such as CO, without requiring recovery means such asa heater. Another object of the present invention is to provide a gassensor which can measure concentration of a catalyst poison gas withoutbeing influenced by H₂O concentration. Still another object of thepresent invention is to provide a gas sensor which has goodresponsiveness.

DISCLOSURE OF THE INVENTION

(1) The invention of claim 1, which solves the above-described problems,is characterized by comprising a proton conductive layer which conductsprotons (H⁺); and first and second electrodes provided in contact withthe proton conductive layer, each of the electrodes includingelectro-chemically active catalyst and being in contact with anatmosphere of an analyte gas, wherein an AC voltage is applied betweenthe first and second electrodes so as to measure an impedance betweenthe first and second electrodes, and a concentration of a catalystpoison gas (concentration of a gas which poisons the catalysts)contained in the analyte gas is obtained on the basis of the impedance.

In the present invention, a change in hydrogen oxidation reaction at thecatalysts with the concentration of a catalyst poison gas is measuredfrom the impedance between the first and second electrodes, which isobtained through application of an AC voltage between the first andsecond electrodes, and the concentration of the catalyst poison gas suchas CO is obtained on the basis of the measured impedance. By virtue ofthis configuration, the concentration of the catalyst poison gas can bemeasured reversibly and continuously with high accuracy and goodresponsiveness.

That is, in a conventional gas sensor which uses a solid polymerelectrolyte (constituting a proton conductive layer) and which obtainsCO concentration from only DC current, since DC current is caused toflow, H₂O is always pumped together with H₂, and the H₂O concentrationin the vicinity of the catalyst of the anode electrode becomes very low.Further, for example, CO having adsorbed onto the catalyst reacts withH₂O so that desorption and adsorption reach an equilibrium state.Therefore, when H₂O decreases, desorption of CO does not occurimmediately even when CO contained in an analyte gas is depleted. Thatis, when CO concentration, which can be obtained on the basis of aCO-concentration-attributable change in hydrogen oxidation reaction atthe catalysts, is measured by use of DC current, the H₂O concentrationin the vicinity of the catalyst of the anode electrode decreases, sothat desorption and adsorption do not reach an equilibrium state, andthus, responsiveness deteriorates.

In contrast, when measurement is performed by use of alternating currentas in the present invention, voltages of alternating polarities areperiodically applied to the electrodes. In this case, since H₂O isalways present in the vicinity of the catalyst, desorption andadsorption of a catalyst poison gas are always in an equilibrium state,and desorption of, for example, CO occurs through reaction with H₂O.Therefore, responsiveness is not deteriorated.

Poisoning by a catalyst poison gas such as CO occurs because theintroduced catalyst poison gas is not desorbed after having adsorbedonto the catalyst. Therefore, through establishment of a condition inwhich a catalyst poison gas can always react as in the presentinvention, occurrence of irreversible poisoning can be avoided.Therefore, concentration of a catalyst poison gas can be reversibly andcontinuously measured without use of recovery means such as a heater.Notably, example waveforms of AC voltage include sinusoidal waveform,triangular waveform, and square waveform.

(2) The invention of claim 2 is characterized by comprising a protonconductive layer which conducts protons; a first electrode provided incontact with the proton conductive layer, the first electrode includingelectro-chemically active catalyst and being shielded from an atmosphereof an analyte gas; and a second electrode provided in contact with theproton conductive layer, the second electrode includingelectro-chemically active catalyst and being in contact with theanalyte-gas atmosphere, wherein an AC voltage is applied between thefirst and second electrodes so as to measure an impedance between thefirst and second electrodes, and a concentration of a catalyst poisongas contained in the analyte gas is obtained on the basis of theimpedance.

In a gas sensor, such as the gas sensor of the present invention, whichutilizes adsorption of a catalyst poison gas onto catalyst anddesorption of the catalyst poison gas therefrom, when the catalystcontents of the electrodes are high, the number of sites at whichdesorption and adsorption of the catalyst poison gas occur is large.Therefore, a long time is needed to create a saturated, equilibriumstate associated with desorption and adsorption of the catalyst poisongas, and responsiveness deteriorates. Further, in the case of a gassensor in which both the electrodes are exposed to an analyte gas,responsiveness depends on the electrode whose catalyst content is high,of the two electrodes. Therefore, a conceivable measure for furtherimproving the responsiveness is sufficiently decreasing the catalystcontents of both the electrodes. However, when the catalyst carryingquantities of the electrodes are reduced, the impedance between theelectrodes increases, so that an SN ratio, which is the ratio betweensensitivity and zero point, deteriorates.

In view of the above, in the present invention, one electrode (firstelectrode) is shielded from an atmosphere of an analyte gas so as toprevent exposure of the electrode to a catalyst poison gas such as CO.Thus, the catalyst content of the first electrode, which is shieldedfrom the analyte gas atmosphere, can be increased, so that deteriorationin the SN ratio does not occur. Further, through reduction of thecatalyst content of the second electrode, which is in contact with theanalyte gas atmosphere, responsiveness can be improved.

Moreover, a change in hydrogen oxidation reaction at the catalyst of thesecond electrode, which is in contact with the analyte gas atmosphere,the change occurring with concentration of a catalyst poison gas, ismeasured from the impedance between the first and second electrodes,which is obtained through application of an AC voltage between the firstand second electrodes, and the concentration of the catalyst poison gassuch as CO is obtained on the basis of the measured impedance. In thiscase, since H₂O is always present in the vicinity of the catalyst of thesecond electrode, desorption of, for example, CO occurs through reactionwith H₂O, so that deterioration in the responsiveness does not occur.

Accordingly, the present invention can provide a gas sensor which isexcellent in terms of responsiveness and which suppresses lowering ofthe SN ratio.

(3) The invention of claim 3 is characterized in that the impedancebetween the first and second electrodes is measured in a state in whicha DC voltage is applied between the first and second electrodes suchthat the first electrode is higher in electrical potential than thesecond electrode.

In the present invention, in a state in which the first electrode isshielded from the analyte gas atmosphere, the DC voltage is appliedbetween the first and second electrodes such that the first electrode ishigher in electrical potential than the second electrode. Therefore, H₂Omolecules accompanied by protons are biased toward the cathode electrode(second electrode), and thus the H₂O concentration in the vicinity ofthe catalyst of the cathode electrode becomes high. Since many H₂Omolecules are always present in the vicinity of the catalyst of thesecond electrode, which serves as a cathode electrode, when CO containedin an analyte gas is depleted, CO having adsorbed onto the catalyst candesorb immediately, so that responsiveness is improved.

(4) The invention of claim 3 is characterized in that the DC voltage isequal to or lower than 1200 mV.

The present invention shows a preferable range of the DC voltage. Whenthe DC voltage is set to a level higher than 1200 mV, the hydrogenconcentration on the first electrode becomes excessively low, so thatcorrosion of carbon and catalyst used in the electrodes occurs.Therefore, the impedance becomes unstable, and responsivenessdeteriorates. Further, durability of the gas sensor deteriorates.Therefore, the above-described range is preferred.

(5) The invention of claim 5 is characterized by comprising a protonconductive layer which conducts protons; a diffusion-rate determiningportion for determining the rate of diffusion of an analyte gas; ameasurement chamber communicating with an atmosphere of the analyte gasvia the diffusion-rate determining portion; a first electrodeaccommodated in the measurement chamber, the first electrode being incontact with the proton conductive layer and includingelectro-chemically active catalyst; and a second electrode providedoutside the measurement chamber, the second electrode being in contactwith the proton conductive layer and including electro-chemically activecatalyst, wherein a DC voltage is applied between the first and secondelectrodes such that the first electrode is higher in electricalpotential than the second electrode, to thereby pump hydrogen orprotons, an AC voltage is applied between the first and secondelectrodes so as to measure an impedance between the first and secondelectrodes, and a concentration of a catalyst poison gas contained inthe analyte gas is obtained on the basis of the impedance.

In the present invention, the concentration of the catalyst poison gascan be detected by measuring the impedance while pumping hydrogen orprotons. That is, in the present invention, a diffusion-rate determiningportion is provided, and a DC voltage is applied between the first andsecond electrodes such that the first electrode is higher in electricalpotential than the second electrode, to thereby pump hydrogen orprotons, whereby the hydrogen concentration in the measurement chamberis lowered. Therefore, in the case where the catalyst poison gas is CO,at the anode electrode side (first electrode side), a shift reaction ofCO caused by H₂O, which is shown in the formula (A) below, isaccelerated, so that CO can react. That is, when the DC voltage betweenthe first and second electrodes is set to a level sufficient for causingCO to react, CO can consistently react in accordance with the formula(A), whereby the catalyst of the anode electrode (first electrode) isprevented from being influenced by CO poisoning.

Through application of an AC voltage between the first and secondelectrodes, a change in hydrogen oxidation reaction at the catalyst ofthe cathode electrode (second electrode), the change occurring withconcentration of a catalyst poison gas, is measured from the impedancebetween the first and second electrodes. According, the concentration ofthe catalyst poison gas can be measured, without being influenced bypoisoning of the electrode by the catalyst poison gas. Moreover, since aDC voltage is applied to the proton conductive layer, H₂O can be pumpedtogether with hydrogen so as to bias H₂O toward the second electrode(cathode electrode). Therefore, the catalyst poison gas and H₂O canalways react on the catalyst of the second electrode, wherebyresponsiveness is improved.CO+H₂O→CO₂+H₂   (A)

(6) The invention of claim 6 is characterized by comprising a protonconductive layer which conducts protons; a diffusion-rate determiningportion for determining the rate of diffusion of an analyte gas; ameasurement chamber communicating with an atmosphere of the analyte gasvia the diffusion-rate determining portion; a first electrodeaccommodated in the measurement chamber, the first electrode being incontact with the proton conductive layer and includingelectro-chemically active catalyst; and a second electrode and areference electrode provided outside the measurement chamber, the secondand reference electrodes being in contact with the proton conductivelayer and including electro-chemically active catalyst, wherein, in afirst operation step, a DC voltage is applied between the first andsecond electrodes such that the first electrode is higher in electricalpotential than the second electrode and such that a predeterminedpotential difference is produced between the first electrode and thereference electrode; and in a second operation step, a DC voltage isapplied between the first and second electrodes so as to pump hydrogenor protons, and an AC voltage is applied between the first and secondelectrodes so as to measure an impedance between the first and secondelectrodes; and a concentration of a catalyst poison gas contained inthe analyte gas is obtained on the basis of the impedance obtained inthe second operation step.

In the present invention, operation is performed in two steps; i.e., astep for applying a DC voltage between the first and second electrodessuch that a predetermined potential difference is produced between thefirst electrode and the reference electrode, and a step for applying anAC voltage between the first and second electrodes so as to measure animpedance between the first and second electrodes. Accordingly, thepresent invention can provide effects similar to those attained by theinvention of claim 5. Further, since impedance measurement can beperformed in a state in which the hydrogen concentration of themeasurement chamber has become constant, even when the hydrogenconcentration changes, the concentration of the catalyst poison gas canbe accurately measured.

(7) The invention of claim 7 is characterized in that the secondelectrode serves as the reference electrode, and the second electrodeand the reference electrode are integrated into a single member.

In the present invention, since the second electrode and the referenceelectrode are integrated into a single member, the sensor structure canbe simplified.

(8) The invention of claim 8 is characterized in that the potentialdifference between the first electrode and the reference electrode isequal to or greater than a potential for oxidation of the catalystpoison gas.

When the potential difference between the first electrode and thereference electrode is greater than a potential for oxidation of thecatalyst poison gas as in the present invention, the voltage between thefirst and second electrodes can be made equal to or higher than avoltage at which the catalyst poison gas such as CO is oxidized.Therefore, for example, CO becomes possible to react on the catalyst ofthe first electrode in accordance with the above-described formula (A),whereby occurrence of irreversible poisoning by the catalyst poison gasis prevented.

(9) The invention of claim 9 is characterized in that the potentialdifference between the first electrode and the reference electrode isequal to or higher than 250 mV.

In the present invention, since the potential difference is equal to orhigher than 250 mV, the voltage between the first and second electrodescan be made equal to or higher than a voltage at which the catalystpoison gas is oxidized. Therefore, the catalyst poison gas reacts on thecatalyst of the first electrode, whereby occurrence of irreversiblepoisoning by the catalyst poison gas can be prevented.

In particular, the potential difference between the first electrode andthe reference electrode is preferably set to 400 mV or higher. That is,when the potential difference between the first electrode and thereference electrode is set to 400 mV or higher, all the catalyst poisongas such as CO can be caused to react, whereby occurrence ofirreversible poisoning by CO, etc. can be prevented.

Notably, the upper limit potential is preferably set to a potential nothigher than the dissociation potential of water (e.g., not higher than1000 mV) in order to prevent generation of error at the time ofmeasurement.

(10) The invention of claim 10 is characterized in that the AC voltageis applied between the first and second electrodes so as to measure theimpedance in a state in which a DC voltage is applied between the firstand second electrodes.

The present invention exemplifies a type of voltage (power source)applied between the first and second electrodes. That is, when an ACvoltage is applied between the first and second electrodes so as tomeasure the impedance in a state in which a DC voltage is appliedbetween the first and second electrodes, a reaction as shown in theabove-described formula (A) always occurs on the catalyst of the firstelectrode (anode electrode), so that the concentration of the catalystpoison gas can be obtained without being influenced by poisoning by thecatalyst poison gas.

(11) The invention of claim 11 is characterized in that the DC voltageapplied between the first electrode and the second electrode is equal toor higher than a voltage for oxidation of the catalyst poison gas.

When the DC voltage applied between the first electrode and the secondelectrode is set equal to or higher than the voltage for oxidation ofthe catalyst poison gas as in the present invention, the catalyst poisongas becomes possible to react on the catalyst of the first electrode,whereby occurrence of irreversible poisoning by the catalyst poison gasis prevented.

(12) The invention of claim 12 is characterized in that the DC voltageapplied between the first electrode and the second electrode is equal toor higher than 400 mV.

In the present invention, since the DC voltage applied between the firstelectrode and the second electrode is equal to or higher than 400 mV,the voltage between the first and second electrodes becomes equal to orhigher than a voltage at which the catalyst poison gas is oxidized.Therefore, the catalyst poison gas reacts on the catalyst of the firstelectrode, whereby occurrence of irreversible poisoning by the catalystpoison gas is prevented.

In particular, when a DC voltage of 550 mV or higher is applied betweenthe first electrode and the second electrode, pumping of hydrogen orprotons is accelerated, whereby the hydrogen concentration in themeasurement chamber can be lowered to a sufficient degree. Therefore,all the catalyst poison gas can be caused to react, whereby theconcentration of the catalyst poison gas (e.g., CO gas) can beaccurately measured without being influenced by poisoning by CO, etc.

Notably, the upper limit voltage is preferably set to a voltage nothigher than the dissociation voltage of water (e.g., not higher than1200 mV) in order to prevent generation of error at the time ofmeasurement.

(13) The invention of claim 13 is characterized in that the lower limitvalue of the AC voltage which is applied between the first electrode andthe second electrode in a state in which the DC voltage is appliedbetween the first electrode and the second electrode is equal to orhigher than a voltage for oxidation of the catalyst poison gas.

According to the present invention, the lower limit value of the appliedvoltage is made equal to or higher than the oxidation voltage of thecatalyst poison gas. Therefore, the catalyst poison gas always reacts onthe catalyst of the first electrode, the concentration of the catalystpoison gas (e.g., CO gas) can be accurately measured without beinginfluenced by poisoning by the catalyst poison gas.

(14) The invention of claim 14 is characterized in that the lower limitvalue of the AC voltage is 400 mV or higher.

In the present invention, since the lower limit value of the AC voltageis set to 400 mV or higher, the voltage between the first and secondelectrodes becomes equal to or higher than the oxidation voltage of thecatalyst poison gas, whereby occurrence of poisoning by CO, etc. can beprevented. Notably, the upper limit voltage of the lower limit value ofthe AC voltage is preferably set to a voltage not higher than thedissociation voltage of water (e.g., not higher than 1200 mV) in orderto prevent generation of error at the time of measurement.

(15) The invention of claim 15 is characterized in that a current whichflows upon application of voltage between the first and secondelectrodes is a limiting current.

In the present invention, the hydrogen concentration on the firstelectrode is further lowered through pumping of hydrogen to a degreecorresponding to the limiting current. Therefore, the reaction of theabove-described formula (A) can be caused to occur in a more stablemanner.

In the present invention, an upper limit current to which currentreaches as a result of application of increasing voltage is referred toas “limiting current.” In the present invention, since AC current isapplied between the electrodes, the average of changing current over asingle period is referred to as “limiting current.”

(16) The invention of claim 16 is characterized in that a hydrogenconcentration of the analyte gas is obtained from the limiting current.

Since the above-mentioned limiting current changes with the hydrogenconcentration, the hydrogen concentration can be measured from thelimiting current. That is, a voltage is applied between the first andsecond electrodes such that the first electrode is higher in electricalpotential than the second electrode, hydrogen is dissociated to protonson the first electrode, the protons are pumped toward the secondelectrode via the proton conductive layer, and the protons becomeshydrogen, which is diffused to the analyte gas atmosphere. At that time,the current flowing between the first and second electrodes (limitingcurrent (the average of changing current over a single period)) isproportional to the hydrogen concentration. Therefore, the hydrogenconcentration can be measured through measurement of the current.

(17) The invention of claim 17 is characterized in that the catalystcontained in the first electrode is a catalyst capable of adsorbing thecatalyst poison gas contained in the analyte gas and generating hydrogenor protons through decomposition, dissociation, or reaction with ahydrogen-containing substance.

The present invention exemplifies the catalyst. That is, when thecatalyst as mentioned above is used, the catalyst poison gas such as COcan be caused to react in accordance with, for example, theabove-described formula (A), whereby occurrence of irreversiblepoisoning by CO, etc. can be prevented.

Platinum and/or gold can be used as the catalyst.

High sensor sensitivity can be obtained by use of platinum or gold. Inparticular, use of an alloy or mixture of platinum and gold ispreferred, because the sensor sensitivity becomes higher.

(18) The invention of claim 18 is characterized in that theconcentration of the catalyst poison gas contained in the analyte gas isobtained on the basis of the impedance measured through application ofAC voltages of different frequencies between the first and secondelectrodes.

The impedance between the first and second electrodes changes dependingnot only on the catalyst poison gas, but also on other gases (e.g.,H₂O), temperature, etc. Therefore, the impedance between the first andsecond electrodes is represented by the sum of impedance Z1 whichchanges depending on the catalyst poison gas, and impedance Z2 which isassociated with other components (e.g., H₂O).

Measurable impedance changes depending on the frequency of AC voltageapplied between the electrodes.

For example, when the AC voltage is of a low frequency of about 1 Hz,the total impedance Z1+Z2 can be measured. Meanwhile, the AC voltage isof a high frequency of about 5 Hz, only the impedance Z2 can bemeasured.

Accordingly, the impedance Z1 corresponding only to the concentration ofthe catalyst poison gas is obtained from the difference between theimpedance Z1+Z2 measured at the low frequency and the impedance Z2measured at the high frequency. In this manner, on the basis of theimpedances measured through application of AC voltage at differentfrequencies, the concentration of the catalyst poison gas can beaccurately obtained, while disturbances by H₂O, etc. are eliminated.

In particular, in a system of fuel cells, H₂O concentration changesdepending on operating conditions, and the impedance changesaccordingly. Therefore, performing correction (H₂O correction) foreliminating the above-mentioned disturbances is preferred.

More preferably, the following procedure is employed. The phase anglesof the impedance Z1+Z2 measured at the low frequency and the impedanceZ2 measured at the high frequency are measured so as to obtain therespective real parts and imaginary parts of Z1+Z2 and Z2. Subsequently,the difference between the real part of Z1+Z2 and the real part of Z2and the difference between the imaginary part of Z1+Z2 and the imaginarypart of Z2 are obtained. By use of the differences of the real parts andthe imaginary parts, impedance components are obtained throughcalculation of obtaining respective root-sum-square values. Thus, theimpedance Z1, which is the difference between the impedance Z1+Z2 andthe impedance Z2, can be obtained more accurately.

Notably, here, an example case in which the impedance difference isobtained has been described.

However, correction may be performed through calculation using Z2, andthe correction method is not limited thereto.

(19) The invention of claim 19 is characterized in that the impedancemeasured through application of AC voltages of different frequenciesincludes two impedances which are measured through application of an ACvoltage having a switching waveform composed of alternating waveforms oftwo different frequencies.

In the present invention, since AC voltage having a switching waveformcomposed of alternating waveforms of two different frequencies isapplied, two impedances can be measured simultaneously through use of asingle circuit. Therefore, the apparatus can be simplified.

(20) The invention of claim 20 is characterized in that the impedancemeasured through application of an AC voltages of different frequenciesincludes two impedances which are measured through application of ACvoltage having a composite waveform composed of waveforms of twodifferent frequencies.

In the present invention, since AC voltage having a composite waveformcomposed of waveforms of two different frequencies is applied, as in thecase of the invention of claim 20, two impedances can be measuredsimultaneously through use of a single circuit. Therefore, the apparatuscan be simplified.

(21) The invention of claim 21 is characterized in that one of the twodifferent frequencies falls within a range of 10000 Hz to 100 Hz, andthe other frequency falls within a range of 10 Hz to 0.05 Hz.

The present invention exemplifies frequency ranges in which theabove-mentioned Z2 and Z1+Z2 can be obtained. By use of impedancesmeasured in these frequency ranges, H₂O concentration dependency can becorrected, so that the concentration of the catalyst poison gas such asCO can be accurately measured.

More preferably, one of the two different frequencies is 5 kHz, and theother frequency is 1 Hz.

(22) The invention of claim 22 is characterized in that the AC voltageapplied between the first and second electrodes is 5 mV or higher.

The present invention exemplifies a range of the AC voltage in whichimpedance measurement is possible. Impedance measurement can be properlyperformed when the voltage is set to the voltage range.

The AC voltage is preferably in a range of 5 to 300 mV because thesensitivity becomes high. More preferably, the AC voltage is set to 150mV because the sensitivity becomes the highest.

(23) The invention of claim 23 is characterized in that the catalystused for the second electrode is a catalyst capable of adsorbing thecatalyst poison gas contained in the analyte gas.

The present invention exemplifies the catalyst used for the secondelectrode. When the catalyst as mentioned above is used, the catalystpoison gas such as CO can be properly adsorbed, so that the impedancechanges. Thus, measurement of the catalyst poison gas such as CO becomespossible.

As the catalyst, a catalyst containing at least platinum can beemployed. Use of a catalyst containing platinum enables propermeasurement of the catalyst poison gas such as CO.

(24) The invention of claim 24 is characterized in that the density ofthe catalyst used for the electrodes falls within a range of 0.1 μg/cm²to 10 mg/cm².

The present invention exemplifies the density of the catalyst used forthe electrodes. In the sensor of the present invention in which theimpedance is measured, its sensitivity can be changed by freely changingthe catalyst quantity. Therefore, measurement of the catalyst poison gassuch as CO can be performed in an arbitrary concentration range.

In particular, the density of the catalyst preferably falls within arange of 1 μg/cm² to 1 mg/cm². That is, when the catalyst quantity isexcessively decreased, the zero point increases, so that the SN ratio,which is the ratio between the sensitivity and the zero point,deteriorates. Meanwhile, when the catalyst quantity is excessivelyincreased, the sensitivity lowers, so that the SN ratio deteriorates.Accordingly, when the density of the catalyst is set to fall within thisrange, measurement of the catalyst poison gas such as CO can beperformed without deteriorating the SN ratio.

(25) The invention of claim 25 is characterized in that the catalystpoison gas is CO or a sulfur-containing substance.

The present invention exemplifies the catalyst poison gas whoseconcentration can be measured by use of the gas sensor. That is, CO or asulfur-containing substance (e.g., H₂S) can be properly measured by useof the gas sensor of the present invention.

Further, the gas sensor of the present invention can be used in anatmosphere in which at least a catalyst poison gas such as CO andhydrogen are present.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory cross sectional view showing a gas sensor ofEmbodiment 1;

FIG. 2 is an explanatory cross sectional view showing a gas sensor ofEmbodiment 2;

FIG. 3 is an explanatory cross sectional view showing a gas sensor ofEmbodiment 3;

FIG. 4 is an explanatory cross sectional view showing a gas sensor ofEmbodiment 4;

FIG. 5 is an explanatory cross sectional view showing a gas sensor ofEmbodiment 5;

FIG. 6 is a graph showing change in impedance with change in COconcentration as measured in Experimental Example 1;

FIG. 7 is a graph showing change in impedance with change in COconcentration as measured in Experimental Example 2;

FIG. 8 is a graph showing time-cause change in impedance ratio withchange in CO concentration as measured in Experimental Example 3;

FIG. 9 is a graph showing change in impedance with change in COconcentration as measured in Experimental Example 4;

FIG. 10 is a graph showing change in impedance with change in COconcentration as measured in Experimental Example 5;

FIG. 11 is graph showing the relation between DC voltage Vp and DCcurrent Ip as measured in Experimental Example 6;

FIG. 12 is graph showing the relation between DC voltage Vp and DCcurrent Ip as measured in Experimental Example 6;

FIG. 13 is graph showing the relation between set voltage Vs and DCcurrent Ip as measured in Experimental Example 7;

FIG. 14 is graph showing the relation between set voltage Vs and DCcurrent Ip as measured in Experimental Example 8;

FIG. 15 is a graph showing change in impedance with change in COconcentration as measured in Experimental Example 8;

FIG. 16A is a block diagram for the case where different frequencies areused, and FIG. 16B shows a combined waveform thereof;

FIG. 17A is an another block diagram for the case where differentfrequencies are used, and FIG. 17B shows a combined waveform thereof;

FIG. 18 is a graph showing the relation between measurement frequencyand sensitivity as measured in Experimental Example 9;

FIG. 19 is a graph showing the relation between measurement frequencyand impedance as measured in Experimental Example 9;

FIG. 20 is a graph showing the relation between AC voltage andsensitivity as measured in Experimental Example 10; and

FIG. 21 is a graph showing the relation between CO concentration andimpedance as measured in Experimental Example 11.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, examples (embodiments) of the best mode of the present inventionwill be described.

Embodiment 1

The present embodiment exemplifies a gas sensor used for measurement ofconcentrations of carbon monoxide (CO) and hydrogen contained in a fuelgas for polymer-electrolyte-type fuel cells.

a) First, the structure of the gas sensor of Embodiment 1 will bedescribed with reference to FIG. 1. Notably, FIG. 1 is a longitudinalcross section of the gas sensor.

As shown in FIG. 1, in the gas sensor of the present embodiment,plate-shaped first and second electrodes 3 and 5 are formed on theopposite sides of a plate-shaped proton conductive layer 1 to face eachother. The first and second electrodes 3 and 5 are sandwiched betweenplate-shaped first and second support members 7 and 9. The first andsecond electrodes 3 and 5 are connected to an electric circuit 15 vialead portions 11 and 13, respectively, so as to enable measurement ofthe impedance between the electrodes 3 and 5. These constituent elementswill be described in detail.

The proton conductive layer 1 is preferably formed of a material whichoperates at relatively low temperature, and for example, Nafion(trademark of DuPont), which is a fluorine-based resin, can be employed.No limitation is imposed on the thickness of the proton conductive layer1. In the present embodiment, Nafion 117 film (trade name) is used.

A porous electrode made of carbon and carrying a catalyst such as Pt canbe used as the first and second electrodes 3 and 5. Alternatively, amaterial obtained through mixing Pt black, Pt powder, or the like withNafion solution may be used, and Pt foil or Pt plate may used. Further,an alloy containing a catalyst component may be used. Notably, anycatalyst can be used, so long as a selected catalyst iselectro-chemically active. The electro-chemically active catalyst refersto a catalyst which can electro-chemically adsorb CO and H₂ and oxidizethem.

A first aperture 16 and a second aperture 17 are formed in the firstsupport member 7 and the second support member 9, respectively, so as toexpose the first and second electrodes 3 and 5 to an analyte-gasatmosphere. Each of the first aperture 16 and the second aperture 17preferably has a shape for facilitating gas diffusion, and may becomposed of a single hole or a plurality of holes. Further, a gasdiffusion flow passage may be formed so as to facilitate gas diffusion.

Each of the first and second support members 7 and 9 is preferablyformed of a ceramic such as alumina or an insulating material such asresin. However, the first and second support members 7 and 9 may beformed of a metal such as stainless steel, if they are electricallyinsulated. An operable sensor can be obtained through a simple assemblyin which the two electrodes 3 and 5 are physically sandwiched betweenthe two support members 7 and 9, and thus are brought into contact withthe proton conductive layer 1. Alternatively these elements may bejoined together by means of hot press.

Notably, the outer surfaces (surfaces opposite the proton conductivelayer 1) of the first and second electrodes 3 and 5 are airtightlycovered with the first and second support members 7 and 9, respectively,so that the outer surfaces are exposed to the analyte-gas atmosphereonly through the apertures 16 and 17.

The electric circuit 15 includes an AC power supply 19 for applying ACvoltage between the electrodes 3 and 5; an AC voltmeter 21 for measuringAC voltage (AC effective voltage V) which is the potential differencebetween the electrodes 3 and 5; and an AC ammeter 23 for measuringcurrent (AC effective current I) which flows between the electrodes 3and 5.

Although not illustrated, in the present embodiment, electroniccomponents (e.g., a microcomputer) for calculating an impedance from theAC effective voltage V and the AC effective current I are used.

b) Next, the measurement principle of the gas sensor of the presentembodiment will be described.

When the gas sensor is disposed in a fuel gas, a catalyst poison gassuch as CO having reached the first electrode 3 and the second electrode5 is adsorbed onto respective catalysts of the first electrode 3 and thesecond electrode 5. Therefore, active sites, at which H₂ on thecatalysts are changed to protons, are covered with the catalyst poisongas.

The adsorption and desorption of the catalyst poison gas reach anequilibrium state in the analyte-gas atmosphere, and the number ofcovered active sites depends on the concentration of the catalyst poisongas. That is, since the equilibrium coverage ratio of the active sitesof the catalysts changes depending on the concentration of the catalystpoison gas, the impedance (between the electrodes 3 and 5) stemming froma hydrogen oxidation reaction of “H₂→2H⁺+2e⁻” changes. Therefore, theconcentration of the catalyst poison gas such as CO can be measuredthrough detection of a change in the impedance.

Specifically, the impedance (Z) can be obtained in accordance with thefollowing equation (B) by use of the AC effective voltage V, which isapplied between the first electrode 3 and the second electrode 5 andwhich is measured by means of the AC voltmeter 21, and the AC effectivecurrent I, which flows between the first electrode 3 and the secondelectrode 5 and which is measured by means of the AC ammeter 23.Impedance Z=V/I   (B)

Since the impedance corresponds to the concentration of the catalystpoison gas, the concentration of the catalyst poison gas can be obtainedfrom the impedance by making use of, for example, a map which definesthe relation between impedance and concentration of the catalyst poisongas (e.g., CO).

c) Next, effects of the gas sensor of the present embodiment will bedescribed.

As described above, in the gas sensor of the present embodiment havingthe above-described structure, an AC voltage is applied between theelectrodes 3 and 5, and an impedance is obtained from an AC effectivevoltage V and an AC effective current I measured at that time, wherebythe concentration of the catalyst poison gas can be measured from theimpedance.

In the present embodiment, since the concentration of the catalystpoison gas is obtained by use of impedance generated upon application ofAC voltage, rather than by use of resistance which is obtained from DCcurrent as in the conventional techniques, the gas sensor has anadvantage of excellent responsiveness.

Moreover, since poisoning occurs when the introduced catalyst poison gassuch as CO is not desorbed after having been adsorbed onto the catalyst.Therefore, through establishment of a state in which the catalyst poisongas can always react as in the present invention, occurrence ofirreversible poisoning can be prevented. Therefore, the gas sensor ofthe present embodiment enables reversible, continuous measurement ofconcentration of the catalyst poison gas, without requiring recoverymeans such as a heater.

Embodiment 2

Next, Embodiment 2 will be described; however, descriptions of portionssimilar to those of the above-described Embodiment 1 will be simplified.

a) First, the structure of the gas sensor of Embodiment 2 will bedescribed with reference to FIG. 2. Notably, FIG. 2 is a longitudinalcross section of the gas sensor.

As shown in FIG. 2, as in the gas sensor of Embodiment 1, the gas sensorof the present embodiment has first and second electrodes 33 and 35which are formed on opposite sides of a proton conductive layer 31 toface each other, and the first and second electrodes 33 and 35 aresandwiched between first and second support members 37 and 39. The firstand second electrodes 33 and 35 are connected to an electric circuit 45via lead portions 41 and 43, respectively, so as to enable measurementof the impedance between the electrodes 33 and 35.

In particular, in the present embodiment, the first support member 37and the electric circuit 45 have configurations different from those inEmbodiment 1.

That is, in the present embodiment, although an aperture 47 forestablishing communication between an analyte-gas atmosphere and thesecond electrode 35 is provided in the second support member 39, such anaperture is not provided in the first support member 37, so that thefirst support member 37 isolates the first electrode 33 from theanalyte-gas atmosphere.

The electric circuit 45 includes an AC power supply 49 for applying ACvoltage between the electrodes 33 and 35; a DC power source 51 forapplying DC voltage between the electrodes 33 and 35 (such that thefirst electrode 33 assumes positive polarity); an AC voltmeter 53 formeasuring AC voltage (AC effective voltage V) between the electrodes 33and 35; and an AC ammeter 55 for measuring current (AC effective currentI) which flows between the electrodes 33 and 35.

b) Next, the measurement principle of the gas sensor of the presentembodiment will be described.

When the gas sensor is disposed in a fuel gas, a catalyst poison gassuch as CO having reached the second electrode 35 is adsorbed onto thecatalyst of the second electrode 35. Therefore, active sites, at whichH₂ on the catalysts is changed to protons, are covered with the catalystpoison gas.

As in the case of Embodiment 1, the adsorption and desorption of thecatalyst poison gas reach an equilibrium state in the analyte-gasatmosphere, and the number of covered active sites depends on theconcentration of the catalyst poison gas. That is, since the equilibriumcoverage ratio of the active sites of the catalysts changes depending onthe concentration of the catalyst poison gas, the impedance stemmingfrom the reaction of “H₂→2H⁺+2e⁻” changes. Therefore, the concentrationof the catalyst poison gas such as CO can be measured through obtainmentof a change in the impedance, which is obtained in accordance with theabove-described equation (B) and by use of the AC effective voltage Vand the AC effective current I.

c) Next, effects of the gas sensor of the present embodiment will bedescribed.

The gas sensor of the present embodiment achieves advantageous effectssimilar to those attained by the gas sensor of Embodiment 1. Further,since the first electrode 33 is shielded from the analyte-gasatmosphere, the catalyst content of the first electrode 33 can beincreased, and the catalyst content of the second electrode 35, whichcomes into contact with the analyte-gas atmosphere, can be decreased.Therefore, in the gas sensor of the present embodiment, responsivenesscan be improved, while deterioration of an SN ratio, which is the ratiobetween sensitivity and the zero point, is suppressed.

Moreover, in the present embodiment, DC voltage is applied between thefirst and second electrodes 33 and 35 such that the first electrode 33assumes positive polarity and the second electrode 25 assumes negativepolarity. By virtue of this, a large quantity of H₂O is always presentin the vicinity of the catalyst of the second electrode, which serves asa cathode electrode. Therefore, when, for example, CO contained in theanalyte gas has been depleted, CO having adsorbed onto the catalyst canbe desorbed immediately, so that responsiveness is improved.

Embodiment 3

Next, Embodiment 3 will be described; however, descriptions of portionssimilar to those of the above-described Embodiment 2 will be simplified.

a) First, the structure of the gas sensor of Embodiment 3 will bedescribed with reference to FIG. 3. Notably, FIG. 3 is a longitudinalcross section of the gas sensor.

As shown in FIG. 3, as in the gas sensor of Embodiment 2, the gas sensorof the present embodiment has first and second electrodes 73 and 75which are formed on opposite sides of a proton conductive layer 71 toface each other, and the first and second electrodes 73 and 75 aresandwiched between first and second support members 79 and 81. The firstand second electrodes 73 and 75 are connected to an electric circuit 65via lead portions 61 and 63, respectively, so as to enable measurementof the impedance between the electrodes 73 and 75.

In particular, in the present embodiment, the first support member 79has a configuration which greatly differs from that in Embodiment 2.

In the present embodiment, a diffusion-rate-determining hole 77 isprovided in the first support member 79 so as to determine the rate ofdiffusion of an analyte gas, which is introduced from the outside of thegas sensor into a measurement chamber 83 (in which the first electrode73 is accommodated).

Meanwhile, an aperture 85 similar to that in Embodiment 2 is provided inthe second support member 81. Pumping of protons (H⁺) from the firstelectrode 73 to the second electrode 75 via the proton conductive layer71 is performed.

The electric circuit 65 includes an AC power supply 89 for applying ACvoltage between the electrodes 73 and 75; a DC power source 87 forapplying DC voltage between the electrodes 73 and 75 (such that thefirst electrode 73 assumes positive polarity); an AC voltmeter 91 formeasuring AC voltage (AC effective voltage V) between the electrodes 73and 75; and an ammeter 93 for measuring current (AC effective current Iand DC current) which flows between the electrodes 73 and 75.

b) Next, the measurement principle of the gas sensor of the presentembodiment will be described.

When the gas sensor is disposed in a fuel gas, hydrogen and a catalystpoison gas having reached the first electrode 73 via the diffusion-ratedetermining hole 77 becomes protons upon application of voltage betweenthe first electrode 73 and the second electrode 75, and the protons arepumped toward the second electrode 75 via the proton conductive layer71.

Accordingly, the impedance associated with pumping out of protons isobtained in accordance with the above-described equation (B) and by useof the AC effective voltage V between the first electrode 73 and thesecond electrode 75 and the AC effective current I flowing between thefirst electrode 73 and the second electrode 75.

Since the impedance component associated with proton pumping changeswith the concentration of the catalyst poison gas such as CO, theconcentration of the catalyst poison gas can be obtained throughmeasurement of a change in the impedance component.

Notably, protons which have been generated on the first electrode 73upon application of voltage thereto and pumped to the second electrode75 via the proton conductive layer 71 become hydrogen on the secondelectrode 75, and the thus-produced hydrogen diffuses into theanalyte-gas atmosphere.

c) Next, effects of the gas sensor of the present embodiment will bedescribed.

In the gas sensor of the present embodiment, as described above, theimpedance can be obtained from the AC effective voltage V measured bymeans of the AC voltmeter 91 and the AC effective current I measured bymeans of the ammeter 93, and the concentration of the catalyst poisongas can be obtained from the impedance with high accuracy and highresponsiveness.

Since the state in which CO introduced into the measurement chamber 83can always react is established, occurrence of irreversible poisoning isprevented. This eliminates the necessity of recovery means such as aheater, and enables reversible, continuous measurement of CO.

Moreover, since the current flowing between the first electrode 73 andthe second electrode 75 is the limiting current, the reaction of theabove-mentioned formula (A) can be caused to occur stably. Thus, COconcentration can be measured stably and accurately.

In addition, since the limiting current flowing between the firstelectrode 73 and the second electrode 75 is proportional to theconcentration of hydrogen within the measurement chamber 83, theconcentration of hydrogen within the analyte gas can be obtained fromthe limiting current.

Embodiment 4

Next, Embodiment 4 will be described; however, descriptions of portionssimilar to those of the above-described Embodiment 3 will be simplified.

a) First, the structure of the gas sensor of Embodiment 4 will bedescribed with reference to FIG. 4. Notably, FIG. 4 is a longitudinalcross section of the gas sensor.

As shown in FIG. 4, as in the gas sensor of Embodiment 3, the gas sensorof the present embodiment has a proton conductive layer 101, a firstelectrode 103, a second electrode 105, a diffusion-rate-determining hole107, a first support member 109, a second support member 111, ameasurement chamber 113, an aperture 115, an electric circuit 116, etc.

In particular, in the present embodiment, in addition to the firstelectrode 103 and the second electrode 105, a reference electrode 117 isprovided outside the measurement chamber 113, which accommodates thefirst electrode 103. That is, the reference electrode 117 is disposed ina small chamber 118 provided in the second support member 111, such thatthe reference electrode 117 is in contact with the proton conductivelayer 101 and is separated from the second electrode 105.

The reference electrode 117 is formed so as to reduce the influence ofchange in concentration of hydrogen contained in the analyte gas.Preferably, the reference electrode 117 is caused to serve as aself-generation reference electrode so as to further stabilize thehydrogen concentration at the reference electrode 117. The referenceelectrode 117 serves as a self-generation reference electrode when aconstant small current is caused to flow from the first electrode 103 orthe second electrode 105 to the reference electrode 117, and a portionof hydrogen gas having flown is caused to leak to the outside via apredetermined leak resistant portion (e.g., a very small hole).

In the present embodiment, the electric circuit 116 operates as follows.A DC power source 119 applies DC voltage between the first electrode 103and the second electrode 105. An AC power supply 121 applies AC voltagebetween the first electrode 103 and the second electrode 105. An ACvoltmeter 123 measures AC effective voltage V between the firstelectrode 103 and the second electrode 105. An ammeter 125 measures ACeffective current I and DC current flowing between the first electrode103 and the second electrode 105.

Further, the electric circuit 116 includes a switching element 127 inorder to selectively connect the terminal on the side of the secondelectrode 105 to the terminal on the side of the AC power supply 121 orthe terminal on the side of the ammeter 125; i.e., in order to effectchangeover between a state in which AC voltage is applied and a state inwhich AC voltage is not applied.

In the present embodiment, the DC voltage applied between the firstelectrode 103 and the second electrode 105 is adjusted such that thepotential difference Vs between the first electrode 103 and thereference electrode 117 attains a constant value (e.g., 450 mV) equal toor higher than 400 mV.

b) Next, the operation of the gas sensor of the present embodiment willbe described.

In the present embodiment, through changeover of the switching element127, first and second steps are alternately performed at prescribedintervals so as to measure the concentration of CO gas.

Specifically, in the first step, a sufficiently high DC voltage isapplied between the first electrode 103 and the second electrode 105such that the limiting current flows between the first electrode 103 andthe second electrode 105, whereby the potential difference between thefirst electrode 103 and the reference electrode 117 attains theabove-mentioned constant value. In this state, current flowing betweenthe first electrode 103 and the second electrode 105 is measured.

That is, in the present embodiment, since the DC voltage applied betweenthe first electrode 103 and the second electrode 105 can be changed suchthat the potential difference between the first electrode 103 and thereference electrode 117 becomes constant, optimal DC voltage is appliedbetween the first electrode 103 and the second electrode 105.Specifically, when the resistance between the first electrode 103 andthe second electrode 105 increases because of, for example, a change inthe temperature of the analyte gas, a higher voltage is applied betweenthe first electrode 103 and the second electrode 105; and when theresistance between the first electrode 103 and the second electrode 105decreases, a lower voltage is applied between the first electrode 103and the second electrode 105.

Meanwhile, in the second step, while the above-described optimal DCvoltage is applied between the first electrode 103 and the secondelectrode 105 to thereby pump hydrogen or protons, AC voltage is appliedthereto so as to measure the impedance between the first electrode 103and the second electrode 105.

Accordingly, the present embodiment achieves not only the effects of theabove-described Embodiment 3, but also the following effect. Throughrepeated and alternating execution of the first and second steps, theimpedance between the first electrode 103 and the second electrode 105can be measured with the hydrogen concentration within the measurementchamber 117 maintained constant and without being affected bydisturbances, and the concentration of the catalyst poison gas such asCO can be accurately detected on the basis of the impedance.

Embodiment 5

Next, Embodiment 5 will be described; however, descriptions of portionssimilar to those of the above-described Embodiment 4 will be simplified.

a) First, the structure of the gas sensor of Embodiment 5 will bedescribed with reference to FIG. 5. Notably, FIG. 5 is a longitudinalcross section of the gas sensor.

As shown in FIG. 5, as in the gas sensor of Embodiment 4, the gas sensorof the present embodiment has a proton conductive layer 131, a firstelectrode 133, a second electrode 135, a diffusion-rate-determining hole137, a first support member 139, a second support member 141, ameasurement chamber 143, an aperture 145, an electric circuit 146, etc.In particular, the present embodiment is characterized in that thesecond electrode 135 has a function of a reference electrode and isintegrated with a reference electrode.

In the present embodiment, the electric circuit 146 operates as follows.A DC power source 147 applies DC voltage between the first electrode 133and the second electrode 135. An AC power supply 148 applies AC voltagebetween the first electrode 133 and the second electrode 135. An ACvoltmeter 150 measures AC effective voltage V between the firstelectrode 133 and the second electrode 135. An ammeter 153 measures ACeffective current I flowing between the first electrode 133 and thesecond electrode 135.

Further, the electric circuit 146 includes a first switching element 149and a second switching element 151. The first switching element 149selectively connects the common terminal on the side of the secondelectrode 135 to the terminal (A terminal) on the side of the firstelectrode 133 or the terminal (B terminal) on the side of the DC powersource 147. The second switching element 151 selectively connects thecommon terminal on the side of the second electrode 135 (the positiveside of the DC power source 147) to the terminal (C terminal) on theside of the ammeter 153 or the terminal (D terminal) on the side of theAC power supply 148.

In the present embodiment, the DC voltage applied between the firstelectrode 133 and the second electrode 135 serving as a referenceelectrode is adjusted such that the potential difference Vs between thefirst electrode 133 and the second electrode 135 becomes a constantvalue (e.g., 450 mV) equal to or higher than 400 mV.

b) Next, the operation of the gas sensor of the present embodiment willbe described.

The potential difference (Vs) between the first electrode 133 and thesecond electrode 135 is measured in a state in which the common terminalof the first switching element 149 is connected to the A terminal.

Subsequently, the first switching element 149 is switched such that itscommon terminal is connected to the B terminal, and the common terminalof the second switching element 151 is connected to the C terminal. Inthis state, DC voltage is applied between the first electrode 133 andthe second electrode 135 such that the measured potential differencebetween the first electrode 133 and the second electrode 135 becomes aconstant value (e.g., 450 mV).

After elapse of a predetermined time, the second switching element 151is switched such that its common terminal is connected to the D terminalso as to apply AC voltage between the first electrode 133 and the secondelectrode 135, while the previously-mentioned DC voltage is appliedthereto. In this state, the impedance between the first electrode 133and the second electrode 135 is measured by use of the above-mentionedimpedance analyzer.

Since the impedance between the first electrode 133 and the secondelectrode 135 changes depending on the concentration of the catalystpoison gas within the analyte gas, the concentration of the catalystpoison gas such as CO can be detected from the impedance.

Accordingly, the present embodiment achieves not only the effects of theabove-described Embodiment 4, but also an advantageous effect such thatthe structure of the sensor can be simplified.

Next, experimental examples performed for confirming the effects of thepresent invention will be described.

EXPERIMENTAL EXAMPLE 1

First, an experimental example performed for confirming the effects ofEmbodiment 1 will be described.

In Experimental Example 1, CO concentration measurement was performed byuse of the gas sensor of Embodiment 1 shown in FIG. 1.

Specifically, impedance measurement was performed under the conditionsdescribed below by use of an impedance analyzer (SI 1260IMPEDANCE/GAIN-PHASE ANALYZER, PRODUCT OF SOLARTRON).

<<Measurement Conditions>>

Gas component: CO=0→2→5→10→20→50→100→50→20→10→5→2→0 ppm

Remaining gas components: H₂=35%; CO₂=15%; H₂O=25%; and N₂ (balance)(volume %)

Gas temperature: 80° C.

Gas flow rate: 10 L/min

Electrode catalyst of the first electrode: Pt carrying carbon catalyst(catalyst density: 15 μg/cm²)

Electrode catalyst of the second electrode: Pt carrying carbon catalyst(catalyst density: 15 μg/cm²)

21 <Impedance Analyzer>>

The following is set between the first and second electrodes.

DC voltage: 0 mV

AC voltage: 150 mV (effective value)

Measurement frequency: 1 Hz

FIG. 6 shows the results. As is apparent from FIG. 6, the sensor output(the absolute value of the impedance Z) changes with change in COconcentration, and therefore, CO concentration can be reversiblymeasured by use of the gas sensor of Embodiment 1, without use ofrecovery means such as a heater.

EXPERIMENTAL EXAMPLE 2

In Experimental Example 2, CO concentration measurement was performed byuse of the gas sensor of Embodiment 2 shown in FIG. 2.

Specifically, measurement of the impedance Z was performed under theconditions described below by use of the above-mentioned impedanceanalyzer.

<<Measurement Conditions>>

Gas component: CO=0→2→5→10→20→50→100→50→20→10→5→2→0 ppm

Remaining gas components: H₂=35%; CO₂=15%; H₂O=25%; and N₂ (balance)(volume %)

Gas temperature: 80° C.

Gas flow rate: 10 L/min

Electrode catalyst of the first electrode: Pt carrying carbon catalyst(catalyst density: 1 mg/cm²)

Electrode catalyst of the second electrode: Pt carrying carbon catalyst(catalyst density: 15 μg/cm²)

<<Impedance Analyzer>>

The following is set between the first and second electrodes.

DC voltage: 700 mV

AC voltage: 150 mV (effective value)

Measurement frequency: 1 Hz

FIG. 7 shows the results. As is apparent from FIG. 7, the sensor output(the absolute value of the impedance Z) changes with change in COconcentration, and therefore, CO concentration can be reversiblymeasured by use of the gas sensor of Embodiment 2, without use ofrecovery means such as a heater.

EXPERIMENTAL EXAMPLE 3

In Experimental Example 3, an experiment was performed to determine theresponsiveness of the gas sensor of Embodiment 2 shown in FIG. 2.

Specifically, the DC current applied between the first and secondelectrodes was changed under the following conditions, impedancemeasurement was performed by use of the above-mentioned impedanceanalyzer, and an impedance ratio was obtained. Notably, impedance ratiorefers to an impedance value normalized such that the impedance at CO=0ppm is set to zero, and the sensitivity (a value obtained by subtractingthe impedance at CO=0 ppm from the impedance at CO=100 ppm) is taken as1.

<<Measurement Conditions>>

Gas component: CO=0→100→0 ppm

Remaining gas components: H₂=35%; CO₂=15%; H₂O=25%; and N₂ (balance)(volume %)

Gas temperature: 80° C.

Gas flow rate: 10 L/min

Electrode catalyst of the first electrode: Pt carrying carbon catalyst(catalyst density: 1 μg/cm²)

Electrode catalyst of the second electrode: Pt carrying carbon catalyst(catalyst density: 15 μg/cm²)

<<Impedance Analyzer>>

The following is set between the first and second electrodes.

DC voltage: 0, 400, 700, 1000, 1200 mV (examples), −100, 1500 mV(comparative examples)

AC voltage: 150 mV (effective value)

Measurement frequency: 1 Hz

FIG. 8 shows the results. In FIG. 8, the horizontal axis representstime, and the vertical axis represents impedance ratio, and FIG. 8 showsa response at the time when CO concentration was changed from 0 ppm to100 ppm. Notably, when a DC voltage of −100 mV is applied, the firstelectrode becomes the negative electrode.

FIG. 8 shows that in the case of a first comparative example in whichthe DC voltage is −100 mV, the response characteristic deteriorates.This deterioration occurs for the following reason. When the DC voltageis -100 mV, hydrogen is pumped toward the shielded first electrode, sothat the H₂O concentration in the vicinity of the catalyst of the secondelectrode in contact with the analyte-gas atmosphere decreases, anddesorption of CO becomes less likely to occur. This reveals that it ispreferred not to apply DC voltage between the first and secondelectrodes (0 mV) or to apply DC voltage such that the first electrodeassumes positive polarity.

Further, FIG. 8 shows that in the case of a second comparative examplein which the DC voltage is 1500 mV, the response characteristic greatlydeteriorates. This deterioration occurs for the following reason. Sincethe hydrogen concentration on the first electrode becomes excessivelylow as a result of application of high voltage, corrosion of carbon andcatalyst used in the electrodes occurs, and the impedance becomesunstable.

The above results show that a preferable range of DC voltage in which COconcentration can be measured by use of the gas sensor of Embodiment 2with high responsiveness is 0 to 1200 mV.

EXPERIMENTAL EXAMPLE 4

In Experimental Example 4, CO concentration measurement was performed byuse of the gas sensor of Embodiment 3 shown in FIG. 3.

Specifically, measurement of the impedance Z was performed under theconditions described below by use of the above-mentioned impedanceanalyzer.

<<Measurement Conditions>>

Gas component: CO=1000, 5000, 10000, 15000, 20000 ppm

Remaining gas components: H₂=35%; CO₂=15%; H₂O=25%; and N₂ (balance)(volume %)

Gas temperature: 80° C.

Gas flow rate: 10 L/min

Electrode catalyst of the first electrode: Pt—Au carrying carboncatalyst (catalyst density: 1 mg/cm²)

Electrode catalyst of the second electrode: Pt carrying carbon catalyst(catalyst density: 1 mg/cm²)

<<Impedance Analyzer>>

The following is set between the first and second electrodes.

DC voltage: 700 mV

AC voltage: 150 mV (effective value)

Measurement frequency: 1 Hz

FIG. 9 shows the results. As is apparent from FIG. 9, the sensor outputchanges with change in CO concentration, and therefore, CO concentrationcan be measured by use of the gas sensor of Embodiment 3.

EXPERIMENTAL EXAMPLE 5

In Experimental Example 5, an experiment was performed to determine theresponsiveness of the gas sensor of Embodiment 3 shown in FIG. 3.

Specifically, the DC current applied between the first and secondelectrodes was changed under the following conditions, impedancemeasurement was performed by use of the above-mentioned impedanceanalyzer, and an impedance ratio was obtained.

<<Measurement Conditions>>

Gas component: CO=1000→5000→10000→15000→20000→15000→10000→5000→1000 ppm

Remaining gas components: H₂=35%; CO₂=15%; H₂O=25%; and N₂ (balance)(volume %)

Gas temperature: 80° C.

Gas flow rate: 10 L/min

Electrode catalyst of the first electrode: Pt—Au carrying carboncatalyst (catalyst density: 1 mg/cm²)

Electrode catalyst of the second electrode: Pt carrying carbon catalyst(catalyst density: 1 mg/cm²)

<<Impedance Analyzer>>

The following is set between the first and second electrodes.

DC voltage: 700 mV

AC voltage: 150 mV (effective value)

Measurement frequency: 1 Hz

Data sampling interval: 5 sec

FIG. 10 shows the results. As is understood from FIG. 10, the sensoroutput changes reversibly with change in CO concentration. That is, theresult shows that CO concentration can be measured reversibly by use ofthe gas sensor of Embodiment 3, without use of recovery means such as aheater.

The electrode catalyst used for the first electrode contains Pt and Auat a weight ratio of 1:1, which are carried by carbon powder. The addedgold may be subjected to an alloying process, or may be contained as amixture.

EXPERIMENTAL EXAMPLE 6

In Experimental Example 6, an experiment was performed to determine therange of DC voltage, in which range CO concentration can be measured byuse of the gas sensor of Embodiment 3 shown in FIG. 3.

Specifically, the DC voltage (Vp) applied between the first and secondelectrodes was changed under the following conditions, and the current(Ip) flowing between the electrodes at that time was measured. In thisexperiment, AC voltage was not applied to the first and secondelectrodes.

<<Measurement Conditions>>

Gas component: CO=0, 20000 ppm

Remaining gas components: H₂=35%; CO₂=15%; H₂O=25%; and N₂ (balance)(volume %)

Gas temperature: 80° C.

Gas flow rate: 10 L/min

Applied Voltage Vp: 0 to 1000 mV (100 mV/min sweep application)

Electrode catalyst of the first electrode: Pt—Au carrying carboncatalyst (catalyst density: 1 mg/cm²)

Electrode catalyst of the second electrode: Pt carrying carbon catalyst(catalyst density: 1 mg/cm²)

FIGS. 11 and 12 show the results. In these drawings, the horizontal axisrepresents applied voltage Vp, and the vertical axis represents currentvalue Ip.

From these drawings, it is understood that in the case of CO=0 ppm, thecurrent value (Ip) becomes constant (limiting current) when the appliedvoltage (Vp) reaches 100 mV. However, in the case of CO=20000 ppm, thecurrent value is low (does not reach the limiting current), which showsthat the sensor has been poisoned by CO. However, in a region in whichVp is 400 mV or higher, the current value starts to increase, and in aregion in which Vp is 550 mV or higher, the current value is maintainedat the limiting current even in the case where CO=20000 ppm.

Accordingly, from this experiment, it is understood that when the DCvoltage is set to 400 mV or higher as shown in FIG. 11, CO starts to beoxidized in accordance with the above-described formula (A), and COconcentration can be stably measured, without being influenced bypoisoning. Moreover, it is understood that when the DC voltage is set to550 mV or higher as shown in FIG. 12, all CO can react in accordancewith the above-described formula (A), and CO concentration can be stablymeasured, without being influenced by CO poisoning.

EXPERIMENTAL EXAMPLE 7

In Experimental Example 7, an experiment was performed to determine therange of the potential difference between the reference electrode andthe first electrode, in which range CO concentration can be stablymeasured by use of the gas sensor of Embodiment 4 shown in FIG. 4.

Specifically, the DC voltage (Vp) applied between the first and secondelectrodes was changed, while the potential difference (Vs) between thereference electrode and the first electrode was monitored; and the DCcurrent (Ip) flowing between the first and second electrodes wasmeasured. In this experiment, AC voltage was not applied to the firstand second electrodes.

<<Measurement Conditions>>

Gas component: CO=0, 20000 ppm

Remaining gas components: H₂=35%; CO₂=15%; H₂O=25%; and N₂ (balance)(volume %)

Gas temperature: 80° C.

Gas flow rate: 10 L/min

Applied Voltage Vp: 0 to 1000 mV (100 mV/min sweep application)

Electrode catalyst of the first electrode: Pt—Au carrying carboncatalyst (catalyst density: 1 mg/cm²)

Electrode catalyst of the second electrode: Pt carrying carbon catalyst(catalyst density: 1 mg/cm²)

FIGS. 13 and 14 show the results. From these drawings, it is understoodthat in the case of CO=0 ppm, the current value (Ip) becomes constant(limiting current) when Vs reaches 100 mV. However, in the case ofCO=20000 ppm, the current value is low (does not reach the limitingcurrent), which shows that the sensor has been poisoned by CO.

However, in a region in which Vs is 250 mV or higher (the region inwhich CO can be oxidized; see FIG. 13), the current value starts toincrease, and in a region in which Vs is 400 mV or higher (the region inwhich measurement can be stably performed without being influenced bypoisoning; see FIG. 14), the current value is maintained at the limitingcurrent even in the case where CO=20000 ppm.

Accordingly, from this experiment, it is understood that when thevoltage Vs is set to 250 mV or higher, CO starts to be oxidized inaccordance with the above-described formula (A), and CO concentrationcan be stably measured, without being influenced by poisoning.

Moreover, it is understood that when the voltage Vs is set to 400 mV orhigher as shown in FIG. 14, all CO can react in accordance with theabove-described formula (A), and CO concentration can be stablymeasured, without being influenced by CO poisoning.

EXPERIMENTAL EXAMPLE 8

In Experimental Example 8, CO concentration measurement was performed byuse of the gas sensor of Embodiment 3 shown in FIG. 3, and COconcentration correction was performed during the measurement.

The concentration of H₂O contained in the analyte gas changes dependingon the operating conditions, and the above-described impedance (inparticular, the internal impedance of the proton conductive layer)changes with the changing H₂O concentration. The correction for COconcentration measurement is performed so as to eliminate the influenceof the H₂O concentration.

In this experiment, impedance measurement was performed under thefollowing conditions. That is, impedance measurement was performed whilethe frequency of the applied AC voltage was set to different frequencies(1 Hz and 5 kHz in cases (1) and (2), respectively, which will bedescribed below).

<<Measurement Conditions>>

Gas component: CO=1000, 5000, 10000, 15000, 20000 ppm

Remaining gas components: H₂=35%; CO₂=15%; H₂O=15, 20, 25, 30, 35%; andN₂ (balance) (volume %)

Gas temperature: 80° C.

Gas flow rate: 10 L/min

Electrode catalyst of the first electrode: Pt—Au carrying carboncatalyst (catalyst density: 1 mg/cm²)

Electrode catalyst of the second electrode: Pt carrying carbon catalyst(catalyst density: 1 mg/cm²)

<<(1) Impedance Analyzer>>

The following is set between the first and second electrodes.

DC voltage: 700 mV

AC voltage: 150 mV (effective value)

Measurement frequency: 1 Hz

FIG. 15 shows the results. As is apparent from FIG. 15, at each H₂Oconcentration, the impedance (accordingly, the sensor output) changeswith CO concentration, and therefore, CO concentration can be measuredat each H₂O concentration.

However, when only data obtained at 1 Hz are used, CO concentrationmeasurement is influenced by H₂O concentration, because the sensoroutput changes with H₂O concentration. Accordingly, as described below,the internal impedance of the proton conductive layer (the impedancebetween the first and second electrodes) was further measured, while themeasurement frequency was changed.

<<(2) Impedance Analyzer>>

The following is set between the first and second electrodes.

DC voltage: 700 mV

AC voltage: 150 mV (effective value)

Measurement frequency: 5 kHz

The results are shown in the following Table 1. In Table 1, thedifference between each pair of impedances measured at the respectivefrequencies is shown. TABLE 1 CO H₂O Difference between concentra-concentra- Impedance Impedance 1 Hz impedance and tion [ppm] tion [%] at1 Hz at 5 kHz 5 kHz impedance 1000 15 38.90 15.80 23.10 20 33.80 10.8023.00 25 31.26 8.18 23.08 30 29.73 6.63 23.10 35 29.14 5.42 23.72 500015 47.84 15.81 32.02 20 42.72 10.76 31.96 25 40.11 8.12 31.99 30 38.586.53 32.05 35 37.88 5.34 32.54 10000 15 51.20 15.88 35.33 20 45.73 10.7435.00 25 43.03 8.06 34.97 30 41.63 6.47 35.16 35 40.51 5.28 35.24 1500015 52.66 15.93 36.73 20 47.47 10.73 36.74 25 44.41 8.03 36.38 30 42.426.42 36.00 35 41.85 5.23 36.62 20000 15 53.77 16.00 37.77 20 48.18 10.7337.46 25 45.09 8.00 37.09 30 43.57 6.32 37.25 35 42.49 5.19 37.30

As shown in Table 1, when CO concentration measurement is performed byuse of only the impedance (A_(1Hz)) between the first and secondelectrodes as measured at 1 Hz, the measurement is influenced by H₂Oconcentration. However, the difference ΔZ between the impedance(Z_(1Hz)) between the first and second electrodes as measured at 1 Hzand the internal impedance (Z_(5kHz)) of the proton conductive layer asmeasured at 5 kHz corresponds to CO concentration.

Accordingly, use of the impedance difference AZ enables accuratemeasurement of CO concentration, without any dependency on H₂Oconcentration.

Here, there will be described two methods a) and b) for measuring theimpedance through use of alternating voltage having a waveform includingcomponents of two different frequencies.

a) As shown in FIG. 16A, in an electric circuit, an AC voltage having awaveform which contains a low frequency (1 Hz) component and a highfrequency (5 kHz) component (see FIG. 16B) is produced throughchangeover of a switch, and is applied to the sensor. The current valueat the time when each of the frequency components is applied to thesensor is converted to a voltage by means of a corresponding IVconversion circuit. The bottom peak of the low frequency voltage and thebottom peak of the high frequency voltage are held, and the impedance atthe low frequency and the impedance at the high frequency are calculatedfrom these values.

A predetermined calculation is performed by use of the impedance at thelow frequency and the impedance at the high frequency, whereby theabove-mentioned impedance difference ΔZ is obtained. After that, a COconcentration corresponding to ΔZ is obtained. Thus, a sensor outputhaving undergone correction for H₂O concentration is obtained.

b) Alternatively, as shown in FIG. 17A, a composite wave composed of alow frequency (1 Hz) wave and a high frequency (5 kHz) wave; i.e., acomposite voltage composed of a low frequency (1 Hz) AC component, and ahigh frequency (5 kHz) AC component superposed thereon (see FIG. 17B) isproduced, and is applied to the sensor. The current value at the timewhen the composite voltage is applied to the sensor is converted to avoltage by means of an IV conversion circuit. The bottom peaks of lowfrequency voltage and high frequency voltage, which are separated fromthe voltage by means of a low-pass filer and a high-pass filter,respectively, are held, and the impedance at the low frequency and theimpedance at the high frequency are calculated from these values.

A predetermined calculation is performed by use of the impedance at thelow frequency and the impedance at the high frequency, whereby theabove-mentioned impedance difference ΔZ is obtained. After that, a COconcentration corresponding to ΔZ is obtained. Thus, a sensor outputhaving undergone correction for H₂O concentration is obtained.

EXPERIMENTAL EXAMPLE 9

In Experimental Example 9, experiments were performed to determine therange of the above-described two frequencies, in which ranges correctionfor H₂O concentration can be performed in the gas sensor of Embodiment 2shown in FIG. 2.

Specifically, under the conditions as described below, the impedance forthe case where CO=100 ppm was obtained by use of the above-describedimpedance analyzer, while the measurement frequency was changed. Also,the difference between the impedance for the case where CO=100 ppm andthe impedance for the case where CO=0 ppm was obtained as sensitivity.

<<Measurement Conditions>>

Gas component: CO=0, 100 ppm

Remaining gas components: H₂=35%; CO₂=15%; H₂O=25%; and N₂ (balance)(volume %)

Gas temperature: 80° C.

Gas flow rate: 10 L/min

Electrode catalyst of the first electrode: Pt carrying carbon catalyst(catalyst density: 1 mg/cm²)

Electrode catalyst of the second electrode: Pt carrying carbon catalyst(catalyst density: 0.015 mg/cm²)

<<Impedance Analyzer>>

The following is set between the first and second electrodes.

DC voltage: 700 mV

AC voltage: 150 mV (effective value)

Measurement frequency: 1000000 to 0.1 Hz

FIGS. 18 and 19 show graphs of the measurement results. In FIG. 18, thehorizontal axis represents the measurement frequency, and the verticalaxis represents the sensitivity at 100 ppm. In FIG. 19, the horizontalaxis represents the measurement frequency, and the vertical axisrepresents the impedance at 100 ppm.

From FIG. 18, low-frequency side frequencies preferable for performanceof H₂O concentration correction can be determined among differentfrequencies. That is, as is understood from FIG. 18, sensitivity isobtained in a range of 10 Hz or lower. Therefore, in the case of the gassensor of Embodiment 2, the frequency suitable for measurement of COconcentration is 10 Hz or lower. Moreover, in consideration of the factthat when the frequency is excessively low, the sampling time becomestoo long with a resultant deterioration in responsiveness, thelow-frequency-side frequency is preferably set to 10 Hz to 0.05 Hz, morepreferably set to 1 Hz.

Meanwhile, from FIG. 19, high-frequency side frequencies preferable forperformance of correction for H₂O concentration can be determined amongdifferent frequencies. That is, as is understood from FIG. 19, theimpedance does not change at frequencies equal to or higher than 100 Hz.Therefore, use of a frequency equal to or higher than 100 Hz enablesmeasurement of the impedance of the proton conductive layer, and enablescorrection for H₂O concentration. The high-frequency-side frequency ispreferably set to 100000 Hz to 100 Hz, more preferably set to 5 kHz.

EXPERIMENTAL EXAMPLE 10

In Experimental Example 10, an experiment was performed to determine ACvoltage for impedance measurement in the gas sensor of Embodiment 2shown in FIG. 2.

Specifically, under the conditions as described below, the sensitivitywhen CO of 100 ppm was introduced (the difference between the impedancefor the case where CO=100 ppm and the impedance for the case where CO=0ppm) was measured, while the AC voltage was changed.

<<Measurement Conditions>>

Gas component: CO=0, 100 ppm

Remaining gas components: H₂=35%; CO₂=15%; H₂O=25%; and N₂ (balance)(volume %)

Gas temperature: 80° C.

Gas flow rate: 10 L/min

Electrode catalyst of the first electrode: Pt carrying carbon catalyst(catalyst density: 1 mg/cm²)

Electrode catalyst of the second electrode: Pt carrying carbon catalyst(catalyst density: 0.015 mg/cm²)

<<Impedance Analyzer>>

The following is set between the first and second electrodes.

DC voltage: 0 mV

AC voltage: 5, 10, 100, 150, 200, 300, 500 mV (effective value)

Measurement frequency: 1 Hz

FIG. 20 shows the results. As is apparent from FIG. 20, impedancemeasurement is possible when the AC voltage is 5 mV or higher. Sincehigh sensitivity is preferred, the AC voltage is preferably set to 5 mVto 300 mV, and more preferably set to 150 mV, at which the sensitivitybecomes highest.

EXPERIMENTAL EXAMPLE 11

In Experimental Example 11, an experiment was performed to evaluatechange in the sensitivity of the gas sensor of Embodiment 2 shown inFIG. 2 when the quantity of the catalyst of the second electrode waschanged.

Specifically, under the conditions as described below, the differencebetween the 1 Hz impedance and the 5 kHz impedance was obtained by useof the above-described impedance analyzer.

<<Measurement Conditions>>

Gas component: CO=0, 10, 20, 50, 100, 200, 500, 1000, 2000, 10000, 20000ppm

Remaining gas components: H₂=35%; CO₂=15%; H₂O=25% and N₂ (balance)(volume %)

Gas temperature: 80° C.

Gas flow rate: 10 L/min

Electrode catalyst of the first electrode: Pt carrying carbon catalyst(catalyst density: 1 mg/cm²)

Electrode catalyst of the second electrode: Pt carrying carbon catalyst(catalyst density: 1.5 μg/cm², 15 μg/cm², 150 μg/cm², 1 mg/cm²)

<<Impedance Analyzer>>

The following is set between the first and second electrodes.

DC voltage: 700 mV

AC voltage: 150 mV (effective value)

Measurement frequency: 1 Hz, 5 kHz

FIG. 21 shows the measurement results. As is apparent from FIG. 21, whenthe catalyst quantity is 1 mg/cm², the impedance hardly changes in therange of 10 to 100 ppm. However, when the catalyst quantity is reduced,the impedance changes for CO of low concentration of 10 to 100 ppm. Thatis, the sensor has sensitivity.

Moreover, it is understood from FIG. 21 that the concentration range inwhich the sensor has sensitivity changes depending on the catalystquantity. From this result, it is understood that the measurable rangefor CO concentration can be changed by changing the catalyst quantity ofthe electrodes of the sensor.

Notably, the present invention is not limited to the above-describedembodiments, and may be practiced in various forms without departingfrom the scope of the present invention.

For example, the electrode catalyst used for the first electrode, etc.are not limited to those described in the above-described embodimentsand experimental examples, and any catalyst can be used so long as aselected catalyst can adsorb a catalyst poison gas contained in ananalyte gas, and can generate hydrogen or protons through decomposition,dissociation, or reaction with a hydrogen-containing substance.

Although recovery means such as a heater is not necessarily required inthe present invention, the recovery means such as a heater may beprovided in order to further improve the performance.

INDUSTRIAL APPLICABILITY

The gas sensor of the present invention is suitable for measurement, ina fuel cell, of concentration of a catalyst poison gas, such as CO,sulfur-containing substance, etc. which are contained in fuel gas, andin particular, concentration of CO. The present invention can provide agas sensor which enables reversible, continuous measurement ofconcentration of a catalyst poison gas such as CO, without requiringrecovery means such as a heater. Also, the present invention can providea gas sensor which can measure concentration of a catalyst poison gaswithout being influenced by H₂O concentration. Moreover, the presentinvention can provide a gas sensor which has good responsiveness.

1. A gas sensor characterized by comprising a proton conductive layer which conducts protons; and first and second electrodes provided in contact with the proton conductive layer, each of the electrodes including electro-chemically active catalyst and being in contact with an atmosphere of an analyte gas, wherein an AC voltage is applied between the first and second electrodes so as to measure an impedance between the first and second electrodes, and a concentration of a catalyst poison gas contained in the analyte gas is obtained on the basis of the impedance.
 2. A gas sensor characterized by comprising a proton conductive layer which conducts protons; a first electrode provided in contact with the proton conductive layer, the first electrode including electro-chemically active catalyst and being shielded from an atmosphere of an analyte gas; and a second electrode provided in contact with the proton conductive layer, the second electrode including electro-chemically active catalyst and being in contact with the analyte-gas atmosphere, wherein an AC voltage is applied between the first and second electrodes so as to measure an impedance between the first and second electrodes, and a concentration of a catalyst poison gas contained in the analyte gas is obtained on the basis of the impedance.
 3. A gas sensor as described in claim 2, wherein the impedance between the first and second electrodes is measured in a state in which a DC voltage is applied between the first and second electrodes such that the first electrode is higher in electrical potential than the second electrode.
 4. A gas sensor as described in claim 3, wherein the DC voltage is equal to or lower than 1200 mV.
 5. A gas sensor characterized by comprising a proton conductive layer which conducts protons; a diffusion-rate determining portion for determining the rate of diffusion of an analyte gas; a measurement chamber communicating with an atmosphere of the analyte gas via the diffusion-rate determining portion; a first electrode accommodated in the measurement chamber, the first electrode being in contact with the proton conductive layer and including electro-chemically active catalyst; and a second electrode provided outside the measurement chamber, the second electrode being in contact with the proton conductive layer and including electro-chemically active catalyst, wherein a DC voltage is applied between the first and second electrodes such that the first electrode is higher in electrical potential than the second electrode, to thereby pump hydrogen or protons, an AC voltage is applied between the first and second electrodes so as to measure an impedance between the first and second electrodes, and a concentration of a catalyst poison gas contained in the analyte gas is obtained on the basis of the impedance.
 6. A gas sensor characterized by comprising a proton conductive layer which conducts protons; a diffusion-rate determining portion for determining the rate of diffusion of an analyte gas; a measurement chamber communicating with an atmosphere of the analyte gas via the diffusion-rate determining portion; a first electrode accommodated in the measurement chamber, the first electrode being in contact with the proton conductive layer and including electro-chemically active catalyst; and a second electrode and a reference electrode provided outside the measurement chamber, the second and reference electrodes being in contact with the proton conductive layer and including electro-chemically active catalyst, wherein the gas sensor has a first operation step in which a DC voltage is applied between the first and second electrodes such that the first electrode is higher in electrical potential than the second electrode and such that a predetermined potential difference is produced between the first electrode and the reference electrode, and a second operation step in which a DC voltage is applied between the first and second electrodes so as to pump hydrogen or protons, and an AC voltage is applied between the first and second electrodes so as to measure an impedance between the first and second electrodes, wherein a concentration of a catalyst poison gas contained in the analyte gas is obtained on the basis of the impedance obtained in the second operation step.
 7. A gas sensor as described in claim 6, wherein the second electrode serves as the reference electrode, and the second electrode and the reference electrode are integrated into a single member.
 8. A gas sensor as described in claim 6, wherein the potential difference between the first electrode and the reference electrode is equal to or greater than a potential for oxidation of the catalyst poison gas.
 9. A gas sensor as described in claim 8, wherein the potential difference between the first electrode and the reference electrode is equal to or higher than 250 mV.
 10. A gas sensor as described in 5, wherein the AC voltage is applied between the first and second electrodes so as to measure the impedance in a state in which a DC voltage is applied between the first and second electrodes.
 11. A gas sensor as described in claim 10, wherein the DC voltage applied between the first electrode and the second electrode is equal to or higher than a voltage for oxidation of the catalyst poison gas.
 12. A gas sensor as described in claim 11, wherein the DC voltage applied between the first electrode and the second electrode is equal to or higher than 400 mV.
 13. A gas sensor as described in claim 11, wherein the lower limit value of the AC voltage which is applied between the first electrode and the second electrode in a state in which the DC voltage is applied between the first electrode and the second electrode is equal to or higher than a voltage for oxidation of the catalyst poison gas.
 14. A gas sensor as described in claim 13, wherein the lower limit value of the AC voltage is 400 mV or higher.
 15. A gas sensor as described in claim 5, wherein a current which flows upon application of voltage between the first and second electrodes is a limiting current.
 16. A gas sensor as described in claim 15, wherein a hydrogen concentration of the analyte gas is obtained from the limiting current.
 17. A gas sensor as described in claim 5, wherein the catalyst contained in the first electrode is a catalyst capable of adsorbing the catalyst poison gas contained in the analyte gas and generating hydrogen or protons through decomposition, dissociation, or reaction with a hydrogen-containing substance.
 18. A gas sensor as described in claim 1, wherein the concentration of the catalyst poison gas contained in the analyte gas is obtained on the basis of the impedance measured through application of AC voltages of different frequencies between the first and second electrodes.
 19. A gas sensor as described in claim 18, wherein the impedance measured through application of AC voltages of different frequencies includes two impedances which are measured through application of an AC voltage having a switching waveform composed of alternating waveforms of two different frequencies.
 20. A gas sensor as described in claim 18, wherein the impedance measured through application of an AC voltages of different frequencies includes two impedances which are measured through application of AC voltage having a composite waveform composed of waveforms of two different frequencies.
 21. A gas sensor as described in claim 19, wherein one of the two different frequencies falls within a range of 10000 Hz to 100 Hz, and the other frequency falls within a range of 10 Hz to 0.05 Hz.
 22. A gas sensor as described in claim 1, wherein the AC voltage applied between the first and second electrodes is 5 mV or higher.
 23. A gas sensor as described in claim 1, wherein the catalyst used for the second electrode is a catalyst capable of adsorbing the catalyst poison gas contained in the analyte gas.
 24. A gas sensor as described in claim 1, wherein the density of the catalyst used for the electrodes falls within a range of 0.1 μgg/cm² to 10 mg/cm².
 25. A gas sensor as described in claim 1, wherein the catalyst poison gas is CO or a sulfur-containing substance. 